Optical networks, e.g., telecommunications networks, are formed of numerous devices. Switches, routers, couplers, (de)multiplexers, and amplifiers are commonplace in networks. These devices must be compatible with one another to function properly, i.e., they must be able to receive and transmit compatible signals. For some networks, this compatibility requires that network devices operate on signals within a specified intensity range—a constraint that makes network power level management quite important.
Systems designers often rely upon optical attenuators to properly manage network power levels. These attenuators can be stand-alone or integrated with other devices to controllably set signal intensities. Intensity can be controlled between serial devices like amplifier stages, between parallel devices like switching banks, and even within a single optical device, like an attenuator integrated into an existing wavelength division multiplexing (WDM) device to normalize channel intensities.
For many applications, attenuators are fabricated by suppliers that, in turn, supply optical device manufacturers who assemble the network appliances (switches, routers, etc.). Since different networks may be optimized for different signal intensity levels, suppliers will often make a batch of identical optical devices and then tailor some of them to meet the needs of the device manufacturer, i.e., the particular network.
Variable optical attenuators (VOAs), where the amount of attenuation is adjustable, are known. VOAs are commonly formed of a blocking structure (like a movable absorber or partially reflecting structure) disposed in a free space region between an input waveguide and an output waveguide. The position of the blocking structure within the free space region determines the amount of attenuation. Shutters, mirrors, prisms, and even liquid crystal structures have been used as blocking structures.
Another attenuation method used misaligns fibers via a mechanical spring, a technique that results in significant temperature-dependent instabilities. Axial separation between fiber ends has also been proposed, though the methods require a large displacement and expensive moving parts.
In other forms, people have developed continuous wave attenuation devices formed of two waveguides twisted and fused together to form a bulk switching/attenuation region. Some of these devices also use thermal elements for selective switching and attenuation control. Still others have developed VOAs that use a Faraday rotator or pockel cell-like structure to attenuate based on polarization state.
While these techniques may be useful for some applications, they introduce undesirable manufacturing costs and complexity of operation. Furthermore, the devices are bulky and incompatible with networking environments where space is a major concern. They are also difficult to install within a network and, therefore, can result in substantial network downtime or slowdown. Perhaps even more important, many of these known VOA devices introduce a substantial amount of unintentional and undesirable loss. For example, insertion loss and polarization dependent loss (PDL) greatly limit operation of known VOA devices. Further, known VOAs also exhibit stability problems malfunctioning if moved or jostled during operation.
It is, therefore, desirable to have VOAs that are not overly bulky, do not use extra components, such as partially reflecting elements or thermal switches, are lower in cost to fabricate, and operate with less loss.